Theoretical study of the mechanism of alkane hydroxylation and ethylene epoxidation reactions catalyzed by diiron bis-oxo complexes. The effect of substrate molecules

The hybrid density functional method B3LYP was used to study the mechanism of the hydrocarbon (methane, ethane, methyl fluoride, and ethylene) oxidation reaction catalyzed by the complexes cis-(H₂O)-(NH₂)Fe(u-O)₂ (n²-HCOO)₂Fe(NH₂)(H₂O), I and cis-(HCOO)(Imd)Fe(u-O)₂(n²-HCOO)₂ Fe(Imd)(HCOO) (Imd = Imidazole), I_m, the "small" and "medium" model of compound Q of the methane monooxygenase (MMO). The improvement of the model from "small" to "medium" did not change the qualitative conclusions but significantly changed the calculated energetics. As in the case of methane oxidation reported by the authors previously, the reaction of all the substrates studied here is shown to start by coordination of the substrate molecule to the bridging oxygen atom, O¹ of I, an Fe(IV)-Fe(IV) complex, followed by the H-atom abstraction at the transition state III leading to the bound hydroxy alkyl intermediate IV of Fe(III)-Fe(IV) core. IV undergoes a very exothermic coupling of alkyl and hydroxy groups to give the alcohol complex VI of Fe-(III)-Fe(III) core, from which alcohol dissociates. The H^b-atom abstraction (or C-H bond activation) barrier at transition state III is found to be a few kcal/mol lower for C₂H₆ and CH₃F than for CH₄. The calculated trend in the Hb-abstraction barrier, CH₄ (21.8 kcal/mol) > CH₃F (18.8 kcal/mol) ≥ C₂H₆ (18.5 kcal/mol), is consistent with the C-H^b bond strength in these substrates. Thus, the weaker the C-H^b bond, the lower is the H^b-abstraction barrier. It was shown that the replacement of a H-atom in a methane molecule with a more electronegative group tends to make the Hb-abstraction transition state less "reactant-like". In contrast, the replacement of the H-atom in CH₄ with a less electronegative group makes the H^b-abstraction transition state more "reactant- like". The epoxidation of ethylene by complex I is found to proceed without barrier and is a highly exothermic process. Thus, in the reaction of ethylene with complex I the only product is expected to be ethylene oxide, which is consistent with the experiment.